Private branch intercommunicating telephone system.



N. H. HOLLAND.

PRIVATE BRANCH INTERGOMM UNIGATING TELEPHONE SYSTEM. APPLICATION FILED NOV. 23, 1907.

967,773. Patented Aug. 16, 1910.

UNI-TED *STATESPhl-ENT OFF-ICE";-

fRIirATE BRA'fioH INTERceMMUNIoAmIhe 'TELE'PI'ioI'I'E SYSTEM.

I I s ecineatidn'orLetterhaient'." Pateh tedAug.-16,1910:

' A l cationfil awovihber 2a, "1367; Serial No. 403,607.

To azz w'ho'm alt-may-eonc'emr of circuits end epp aiatne'.whe 1 ehy'the oper Be it known thatLN A H, H LLAND, atlons requlred of the party in transferring" citizen oft-he-United-Statee, residi'hg at Ghiit call reeeive'd'over a. trunk line from one 1 cage, in the county of Ookand"State L01 l cal S ation to anot a si nplified. 55 5'I111nois, 'ha;v;e invented a eertain'-new;',and O ne feature 0f.my':1nven ,t n consists in' useful Improvemenb'in Private BranehxInthe provision of two ringingkeys; one of tercomm'unio'ating- Tele honejsyscem's, f wh ch Is fo u' se whe el c mte 'communi- Which-the following is 'a"ful1,-=o1ea -r, concise, cation onl 1s involved, and the other where and e-xaot dESCI'iPt'IOIJ: a party'a ter receiving a; call over: the trunk 50 M i ti l t {0.3; i te b a h llnedesires to eomr'r'umieate with some other invention relates-more pit set-mg intercomlnunleating- 'system',*' and 'a so pr0-' can 63 Q y .111 addition o vi'dingqior.-outsideconnections With'the cen- 12111169911" as g y, t? to flstfibhsh Haliofficegpxchaflgey 1 1 conditlone'whieh W111 ma ntain the clrcuit 65 u a cont nuity of the trunk hne by closing a k' path in substitution of the Circuit closed exchange -te1eph0ne*"syst'en1;: embed ing an local Station, a example, to transfer the an arrangement"wherebya cell? e line m be answereeet' any foneof several t g h te ep s v StfitiODSi-ll th private-exchange; and Where-- Qthen'featfil es oi: my invention relate to' by the -.te1ephZ1'1e-usedfor answering: an onthe p yt culer strlicture and rr g 70' side 021.11 may be utilized-for communicating the switch keys et the loeal stations, andthe with another local-station Withoiit'eausing a clrcuits contrelled by seijd heys,

disconnect-signalet the central ofiiee during These and other features of my .invehtion the intervalofsuc'h use. may be' more readily understood by referi- I The object oftmy invention is to reduce 191166 PO p nyi g drawin which 75 the number of operations involved in trans- 1S 3' g m of i fl dRPPQi SWh h ferring a; ,ea1i -rreeeiVed" over a trunk line .m y be imp by lf FO Ij sake of iclea fngs i from one station to =an0t'he'i"=- nd to other" onlytw'o stetionsoff the inte feqn iiiinnieqt ng wise sim lify the-'"epei'fjitibn-of th system. ys e and one tfi k1lg1 fie ml net iig 'the e-v My'invention p 'ovi'dee' ineahe at; two or 1n,; are's howi1, -1t bemg'obvlouethatas. many 80,.

more-stationsef'an-intereomfiihifiiating ey llnes inay be employed as maybe tern for m'aintaini-ngthe' ei'rcuit continu ty deemed n ece I i g endin .subst uti fiof -t-he=te1-eph es'et a't fassoe-l d Wit theldeafiiiiesof-.every'oth er said stat-ions Bys'ueh iiif'i'afig' ioi statio i l beingdesigne'ted' I .ere'provided: 0'1 holding' e-a llifig l 'wh'ile j" statio'lil'Ies L". For sake i the telephone-Set Whic1I"WaS'ued m y,-the';penels"of loca lfkeys-ere A 7 ing the eall-is being liti zetfii ing with ametherle'ee'l s' g which bf course; 9 n-idle ke' Wm? I I iEachfeg 'ging Q fi'ent t'o the 'eration of the. l'cal.

. v v the circuit throu%h-"Which is closed at -jthe, 4 My*imientioii'f-eonsists in zen-arrangement switeh contacts-"w en the receiver is on it med iitio i'ito Eegh "epipfii is' providedffi ith ecajrieys "the usual. ba'llibll 5,1160

station. T

l jandfa'dapted' when depressed to connect the telephone set of that station with the corre- 0 opposition to 'a retractile 's ring w. Said;

hook. The battery B furnishes the source. of ringing current. The.- ringing key R is used in ordinary intercommunication from one station to another. The ringing ,key R is a combined transfer and ringlng ke and is employed whenever a callis receiv over a trunk line and the answering station wishes, in order totransfer the call .or for, any other ose, to'callup another local lie unction ofthe transfer ring ng key Rf .will be here'inafter more ful y set forth Each station is also provided .wit a .trunk key'Tassociated with the trunk line,

ressed osition, and, therefore, plate E 'ocks sai' key down. .The same is true as appliedflto the local key L when a station 'is placed in connection with any one-0f the local lines, and hence plate D is provided ,for lockin' saidlocal keys-down. When key T is de ressedv and looked under the control of p ate E, and the call is to be transferred to the line associated, for example, with key L it is desired not tov cause the releasing of key T when thekey L is depressed and locked, for otherwise the con- 'tinuity of the trunk line would be broken. However, after ke' L is locked -down,' and the ringing key R isused to rin the local spending" trunk line. i p y The local keys at, each station are provided with: conical shoulders l which'eng'a e with'a sliding plate D, moving the same 1n thekey T todisconnect the telephoneset at station I from the trunk line, and to main- :tain the key" L in the locked position. keys when=depressed are h dlocked down other means are provided, by the act of d'eby said plate D. I11 a like manner the Pressing the key R, to automatically estabtrunk ke s T and the answering keys are isha locking circuit for the trunk line; in associate with a plate E. -The late E substitution of the telephone set-,- as,- will 5 overlies-the plate D and isfconnecte thereto .hereinafter be more fully described.

9 .the"end ofthe plate E. Theringing ke s R othrplates ,and'willreleaseeach other," but. owing to thefact that thecollar l' of, ,said

k gingkeys" do not release a y-either local l t will-be noted that thekey'arrangement;

cal s receivedjover the trunkffrcin one star bya comparatively 'stiflj' spring a, in'order ;It is evident that should a local key-L 19" that the movement of-the plate E by the be depressed at the tune the party removed :keyswill effect. the movement of plate D. thefelephone from the hook an such'party :The plateD has a'shoulder 01, which engages desires communication "with the trunkline',

' the act ofdepressingkey-T will, cause ;a

release of the local key L, resultant from .95 the movement of plates E and D, the spring being enough greater in? tension than the spring, W' to permit such movement, The converseIof, this'condition would be when a trunk-k y was depressed at the time the and R are ada ted to-mov'e the late but are constructetl so that" they d not lock down "when depressed, Said ringing keys are also 'providedwith an enlarged-collar c forming'a'stop whichfpreventsthe plate D being'moved y the act of depressing either p of sald ringing-keys. ,t tblBPhOIlQlt-Z to be used for communicating 'The' structure of the keysat each station ver one of, the local--li nes; Though the .is, the same; Referring'to station I; it ,will depression of he locallnay-L will notcause be noted that the trunk key T or the answer "w l ngbf: h 't lnkkey thatikey W llbe ingke'y A will l ove.broth-plates D andiE and willrelease any key which lHEtYbQdG- fused in calling the local party t" can ressed. :The local keys L and IE will move readily beseen that the two'keys R and Rf enabl'e'a party todifi'erentiate between wherii it is, nor is not desired" to lock or hold the local keys is smaller than thecollars Z" of the ke s A and T, the local keys will not releasd'te'trunk jkey T or the answering keyA; Both of the rinlgingfkeys RQ'R 5-; move the top plate E on y, thusreleas'in' any trlink key or answer ng-kc AI -whic may be depressed. The stops e 0? said ring ing keys. rev entth'e ring'gng keys from op-'-. .erating, .t elowerjplate andhe'nce said I he 'answerin'g' key-A is associated with :plate 13] so that any key which ,the party a may findde resse'd at 'his- 'stat-ion may be released at the time theans'wering key is Considering first merely theintercommuni'cati ng feature of the system illustrated v o ingthe-"drawling, each looaletatiom as'shown for: example-at station- I, andxstatio'n II, is equippedwith the usualtfilfiP'hOIlQ set-gem, pri i g: transmitter ;t.-ran' a "receiver-7 Correspondin' with seachg-st-ation 'is a local i 1 11116 a-i= an. 2 -1 e a ve y li e J with aijcomnion a 'throughtheusual e ard i i sj x 1 The'mannerinwhichasubscribenat one -sta tionfmay T681111 another station is ohv vious, For, exanip e,'-.' ifthoflsubscriber'fat of my nvention permits thettransferringof tion to anothriw'ith comparatively dfiewjop iterations on the part of the erson answerin Y the trunk call. For examp e, whenfia trun -'call is answered b depressing'key T,-it-,i's desired tohave his :key remain inrafide-r party, It: is desired to automati ca'ly release .80

"Therefore since key Tis released by key It; i

released at the'time gtheringingke R is" 0 I trunk line at-the time a local party is called. 1 0

station I desires to communicate with station ll, he will remove his receiver 1' from its switch hook It, and depress key If, thus closing contacts 3, 4- anrl connecting his telc phone set with local line 2, 2. He will then depress his ringing key R,'llu. rehy closing contacts 5, t3, whereupon, if the called line is not in use, current will liowi'rom l-Jattcry B through conductor 7, contact 5, resistance 8, conductor 9, contact 3 oi' key L conductors 10, 2, 11, contact 12, conductor 1.3, includii'lg bell I), conductor 14, back to battery B. The subscriber at station II will respond by depressing his answerin key A and by removing his receiver '1' T011! its hook. Stations I and II are now in telephonic coninnu'iicatimi, the telephone sets thereof being connected to local line 2, 2-,

through local key L and answering key A'-, respectively. i

The circuit arrangement just described provides-means whereby the calling party is informed Whether or not the station called is busy. At the time the party at station I depresses-his ringing key R, current was supplied forringing the bell. b at station II, said current flowing through resistance 8. One terminal of receiver 1' is connected through conductor 9 to one side of said resistance, and the other terminal of the receiver is connected to the other side of said resistance through conductor 16 and the closed contactv Got the key R. The ringing of the belt-b at station II causes a buzz in the receiver r, due to the diiference in potential across the terminals of resistance 8, this 'resistance being in series with. the bell at station II. The current flowing through resistance 8 is intermittent, owing to the opening and. closing. of. the circuit in the vusual manner in the contact of the bell b. The buzz denotes to the party at station I that the telephone at station II is not in use, sinee'if it were, contact 12 would be open, the bell b at station 11 would not ring,

and the 'party'at station I would not hear a cell's without-fcausing'ai' v be displayed during the-interval of. transfer.

buzz'in his receiver.

' It will be observed that the arrangeni'en thus far described 'in detail provides what is commonly designated as, a'n; intercomniunicating System. :I will describe" the .circuitsl'and arrange/me t 4 system'- iriay bemonnecte t 'ajceiitralof ce l S ide Corri iiuni-' hereby such a -"rovi de f0 eating-and", or penning ransiterring bf nnect s1gna'l to line extending. from r h' central; ofiice excha ngc tofl 'jtl e 'interconimunicating system. ,Said trunk line ismultiplied as shown, with I eachistation of the intercomr'nunicating'sys m. 1 Eachstation is provided, as hereinbestated', witl1 a trunk key 'I' associated learning the conversation or causin with the trunk line, and adapted when dcpressed to connect the telephone set of that station with the trunk line.

Each station is provided, as before (lescrilocd, with a transfer ringing key lt, said key being used. when it is desired to trans fer, from one local station to another, a call received over a trunk line. lhe iunction oi said transfer ringing key, asiur as the ringing circuit is concerned, is the same as the local ringing key It. But the transfer ring ing key R has additional contacts associated with it, which serve to operate a relay holding device -M. Said holdingrelay M is placed. in a bridge 19,- 20, adapted to inaintain the continuity of the signaling circuit established over the trunk line in.

substitution of the telephone set, thus enabling the station at which the trunk call is answered to call up and connnunicate with any other local station without iying a disconnect signal at-the central 0 cc. By pressing down transfer .ringing key It therefore, current is applied to the ringing circuit in the same manner as previously described in connection with the operation of key R, and a bridge is placed across the trunk line, said bridge including relay M. The relay M in said bridge acts as a hold ing coil in substitution of the telephone set for preventing the display of a disconnect signal at the central oflice. It will be noted that the bridge 19, 20, has a normally open contact 21 controlled by relay M and a normally closed cont-actv 22 controlled by relay M The relay M is in series with the trunk line, and therefore, the called party in. making connection with the trunk line after the bridge including relay M aforementioned has been established, will cause the relay M at this time held up through its contact 21, to become 'de'e'nergized; It will thus be seen that thetrunk line is directly under the control of the switch hook contacts of the particular station which has made connection with the trunkline.

A signal bell b may be associatedwith the trunkline in any manner to" give a signal at the station or stations at which it is de sired to answer an incoming, call. The signal bell may be located at some point, for exa1nple,-where 'it can be heard from any station.

' A call over the trunk line may beanswered at any station with which the trunk line is multipled, and the answering stationmay thereafter communicate with any other station without the central oflice operator it disconnect signal at-the central-ofiice.

' more, the answering station may then either renew the conversation, with- .*he callingparty, or request the called local party to "answer over the trunk line.

urthei secondary to the main line circuit.

50 that the; first e The operation of the system as a whole is as follows:' Assume that a call is received over the trunk line, and is'answered at station I. The )arty at said station removes his receiver 'rom its hook and depresses Key '1. will remain locked 32 including relay M, and conductor 18 of the trunk line to central office. This com pletes the circuit continuity of the trunk inc and-establishes conditions at the central ofiiee which show, in the usual manner,

20 that the call has been answered.

At the same time a ath for telephonic currents, in which is inc uded the telephone receiver, has been established, which path is local to the private ranch exchange and is Such path may be traced-as follows: from conductor 18, through condenser G, Winding 33 of induction coil II, conductors 34,35, contact 36 of key T, conductor 9, receiver a, contacts of switch hook 71,, transmitter 15 conduc-- tor 29,'contact 30 of key T, conductors 31 and'32 back to the point where condenser G connects to conductor Station I 1s now so connected to the trunk line that conver' 35 sation' can take place between the private branch exchange party and the calling party, connected through the central exchange, the party at station I controlling the central oflice signals in the usual manner, that is, 40 by means of his switch hook. If the party at station I now Wishes to call up any other station of the intercommunicat-ing system for any purpose, as for vexample,-to receive instructions, or to transfer thecall, he may readily do so. For instance, to call up station II, the party at station I-would depress local key'L corresponding to station II, and p thenring' that. station by depressing the transfer ringiiilig key R It will be noted ect of pressing down key R is to open the contact 27 of said key. Since the circuit of relayM is thus opened at contact 27', said relay becomes'deenergized' and,

contact 22 of said relay closes. Itwill be 5.5 further noted that. the keys. are so adjusted that, thouglrkey It releases .key T, the contacts 25 and 4'0 of key T 10. not open until after contact 37' o-fkey R is closed. I l

The substitute path established for one. rent on the trunk line by the depression o-f' transfer rin ing keys-R may now be'tr'aced as follows: rom conductor-18, through conduct-or 19, relay M, conductors 3-8, 39, con tact 40 01' kc T, conductor'etl, contact 37 of key It, con uctor 26, contact 25, conductor station to answer over the trunk line. party at station II then depresses his trunk 2 1 to conductor 17. Due to the closing of this circuit, the relay M becomes energized, closing contact 21, and remains locked up by current flowing in the bridge closed from conductor 18 through conductor 19, relay M, contacts 21, 22, conduct-or 20 to conductor 17 The bridge thus placed across the trunk line maintains the continuity of the signaling circuit of said trunk line. The release of key T at station I disconnects the telephone set from the trunk line and permits cation is established between stations I and II, in the manner hereinbefore described in connection with the intercommunicating features of thesystem. After communicating withstation II, the party at station I may resume conversation over the trunk line with the outside calling party, or he may instruct the party at station II to answer over the trunk line. In fact' either thep'arty at station I or the party at station II can by de pressing the trunk key T at his station connect his telephone set to the trunk line, the resulting operations being the same in either case.

Let us assume that the party'at station I desires to transfer the call to station II.

After, calling up station II in the manner ust described, he instructs the party at that The e'y T and n; so doin releases his answering key, leaving his telep one set connected only with the trunk line. The party at station II is now in position to talk with the outside calling party. \Vhen the party atstation II answers over the trunk line, as above described, by pressing the, key T, the following circuit con 18, through relay M conductor 32, conductor at, contact 45, conductor 46, trans mitter T, switch hook it, conductor 47, contact 49 of key R conductor 50, contact 51, conductor 52, conductor 17 back to the central oilice exchange.

crates to open ,contact'22.' The brid e; of the trunk line over conductors 19, 20', ing thus open at contact 22', the relayM becomes deenergizecl, thus also opening contact 21 in said bridge.

R, and in the other at contact ltions are established: Current. flows from the central oflice over conductor Relay M being in'the circuit thus completed is energized endop- The receiver circuitat station II is as fol- V lows: from the conductor 13, through co'n- 1-39 'intercoinniunicatin'g system and nullltipied with two or more stations thereof,

a 'col'nlnned ringing and transfer ln-ry v the circuit continuity of the trunk line, and

'nicat'ingtclephone system, of trunk line tiplecl'with two or more stations thereof, a

'nicating teleyhone system, of a trunk line call recei' eri over the. trunlrline, said key in denser G, winding 33 of the induction coil H, conductors 3 1-, 53, contact 54 of key '1, conductor 55, receiver '7', switch hook it, transmitter a, conductor 46, contact 45 of key T, conductors 44, 32, relay M, concluc tor 18 back to the point Where condenser G connects with conductor 18.

It will be noted that the brid e 19, 20, across the trunk line was remove as a rccu lt of the party at station 11 depressing the trunk key T, and therefore the continuity of the trunk line is maintained during the conversation only by the contacts of the switch hooks. When the party at station II J hangs up'thc receiver at the'end of the conversation this continuity is thereby destroyed and th usual disconnect signals given at the central otlice. It will be observed that substantially the samecondi tions apply Whether. :a call received over the trunk line is actually transferred from the station answering the cell to another one of l the local stations or whether the answering F station, Without losing connection with the trunk line, merely talks with another station, for example, for the purpose of ob mining information. Hence it will be 17.11- ed the terms transfer and tranc y, in their broad sense, are applito both of the above-mentioned condiaim:

l, The combination with an intcrconnnn meeting telephone system, of a trunk lineicarling trcm a central ofiice to said intercommunicating eystcui and .inultipled with two or more stations thereof, switching mechanism at said stations for lacing the telephon setthereof into telephonic coinmunicatrn' over trunk line, adclitionel means common to stations and assoelated with trunk line for maintaining switchingmeans at each of said stations for substituting-said additional means for the. telephone set to prevent the disgilay of a disconnect signal at the. central oi ice.

2. The combination with an interc0mmuleading from'a central.,..o,tfice exchange to said 1ntercoinmunicating, syste'mand muL- bridge of thetrunk line common: to said etations, and circuit closing deyicesl at each. ofsaid stations-for substituting said bridge for the telephoneeet of that station to maintain the circuit continuity of saicl trunk line.

3. The clombinetsl-on with an intercom nu leading from a central i'iftice exchange to rlt each of said stations for traneferring-a system havinga trunk, line lea its operation producing circuit conditions for maintaining the circuit continuity of said. trunk line during the interval of the transfer.

4. In an intercommunicatin S YStQIH having a trunk line lea ing to said intercoininunicating system and Inulti pled with two or more stations thereof, switching mechanism at each of said stations for connecting the telephone set of that station to the trunk line, a circuit. in substitute of the circuit of the telephone set, for main- 'taining the continuity of the trunk line, and combined ringing and transfer key having contacts controlling the local ringing circuit of Said interc'ommunicating system, and also having contacts adapted by the operation of said key to procluce circuit conditions for completing the continuity of the trunk line through said substitute circuit.

5. In an interconununicating telephone system having a trunk line leading to said intercommnnicatingsystem and multipled with two or more stations thereof, swztclb ing mechanism at each of said stations"- for connecting the telephone set of that st-a n to a trunk line, a circuit in substitute of circuit of? the telephone set, for maintaining the continuity of the trunk line, a combir. 1 ringing and transfer key having conta controlling the local ringing circuit-of sale intercoimnunicating system, and also havin contacts adapted by the operation of sai: key to produce circuit conditions for coinpleting the continuity lot the trunk line through said substitnte circuit, end-a trunk key at each of said stations having contacts aclaptefi, when said trunk key is depressed, to connect the telephone set of that; station the trunk line hereby circuit conditions are established which Will. cause the subetitnte circuit to'be opened providing the re ceiver is ofi" its hook.

6. In an intercommunicatin system having a trunk line lea ing from a central ofi'ice exchange to said intercoinmunlcatingsystem and, multiplied with two or more stat-ions thereof, atrunk key at each o.f" sai(l stations having contacts controlling the connection of the telephone set of that.

a transfer key at each of said stations; having contacts controlling the transfer from one of said. statimes to another of a call received over the station to the trunk line,

trunk line, a h olding relay for-maintaining the circuit continuity of the trunk line, sat relay having a circuit inclndin a serialize rangelnentof contacts of said lreyga look .ing circuit gfor 'saidrelay closeil upon the energi'zati-on thereof, and means whereby the operation of said. trunk key subsequent to the closure of said locking'circui i 7, In an intercommunicatin Sing from a telephone 7 telephone said transfer key releases telephone central oflice exchange to said intercommuniing a normally closed contact ofthe transfer eating system and multiplied with two or key and a normall on contact of the more stations thereof, a trunk key at each trunk key. at each ot sai stations. K

of said stations having contacts controllin 10. In an intercommunicatin telephone t e connection of the telephone set of that system avlng a trunk line lea 'ng to said 0 station to the trunk line, a transfer key at system and multipled with two or more each of said stations having contacts constations thereof, a trunk key at each of said trolling the transfer from one of said stastations having contacts controlling the contions to another of acallreceived over the nection of the telephone set of that station trunk line, a bridge of said trunk line for to the trunk line, a bridge of the trunk line, maintaining the continuity of the trunk 1n substitute of the telephone set for mairr line during the interval of transfer, and a tainingthe circuit continuit o the trunk relay for controlling the continuity of said line, a holding relay controlling said bridge,

7 trunk line, aholding, relay or maintain bridge, the circuit of sa d relay lncluding an a transfer key at each of said stations a normally closed contact of said transfer for closing a circuit through said holding key anda normally open contact of said relay. trunk key. 11. In an intercommunicatin telephone 8. In an intercommunicating telephone y em avlng a trunk line lea 'ng' to said ste h vi a, t k li l di fr a system and multlpled with two or morestacentral olhce exchange to said intercommuniions thereof, a trunk key at each of Sald nection of the telephone set of that station to the trunk line, abridge of the trunk line, 1n substitute of the telephone set for maintaining the circuit continuity o the trunk line, a holding relay controlling said bridge,

more stations thereof, .a trunk key at each of said stations having'contacts controlling the connection of the telephone set of that station to the tr nk line, a transfer key at lay, and a locking circuit for said relay closed upon the energization thereof.

In an intercomniunicating telephone system having a trunk lmeleading to said system and multlpled with two or more stations thereof, a trunk key at each of said *stationshaving contacts controlling the con: nection of the telephone set of hat station to the trunk line, a bridge of the trunk line, in substitute of the telephone set, for maintaining the circuit continuity of the trunk line, a holding relay controllin said'brid e, a transfer key at each of saif stations for mg the circuit. continuity of the'trunkline, said relay having a circuit 1nclud1ng a serial trunk key subsequent to the closure of said locking circuit, and a second relay cont-rolling the continuity of said locking circuit, the circuit of said second relay including a normally closed contact of said transfer key and a normally open contact of-said trunk key.

9. In an intercommunicating telephone system h in a t k lin h di f a the energizatlonthereof, and a circuit under the control of the trunk he at each of said stations for effecting the dig" said holding relay and removing said bridge more stations thereof, a trunk key at each of thetrunk line.

of said stations having contacts controlling the connection of the telephone set of that station to the trunk line, a transfer key at system having a trunk line lea ing to said 3 trunk line, a holding relay for maintain a bridge of the trunk line, in substitute of mg the clrcuit continuity of the trunk line, the telephone set, for mainta ning the oilsaid relay having a circuit including a serial cult continuity of the trunk line, a holding arrangement of contacts of said keys, a' lockrelay controlling said bridge, and switchln ing c rcuit for said relay closed upon the mechanism at each ofsa1d statlonsfor con trunk key subsequent to the closure of said I ti onof said holding relay.

locking circuit, and a second relay adapted ll. intercoinmunicating telephone sy when energized to open said locking circuit, tem' having: aplural-ity of local stations, a I trunk line I onnected therewith, switching mechanism at each of said stations for join- .5%

ing its telephone set to the trunk line and to I line for rendering the substituted circuit in 10 another local statioe, a circuit which may be substituted for :1 telephone set to maintain the continuity oil: the trunk line, means for effecting such substitution when the stationwith its seteonneeted to the trunk line seeks communication Wifl': another local station, and means operable upon the uniting e? a station. teiephene with the trunk effective.

In Witness wlixel'eo'f, I, hereunto 311350111130 my name thls 12th day of November A, 1).,

NENMAN H. HOLLAND. Wilanessses RALPH G. JOIIANSEN, E. F. BEAUemN. 

